Constant Radiance Research Articles

Light-field spectral decomposition with a spatial–angular consistency prior for disparity estimation

Modeling the spatial–angular relationship is the theoretical basis for accurate disparity estimation from the light-field data. In this paper, the spatial–angular coupling relationship of light-field is established under the assumption of constant radiance along rays, and the spatial–angular consistency model of light-field is established under the assumption of Lambertian radiance. Based on the spatial–angular consistency prior of light-field, the light-field spectral decomposition based on disparity layers can be modeled as the ill-posed inverse problem. Based on the spectral decomposition, the disparity estimation is transformed into the process of searching for the spatial domain support set in the scene disparity hierarchy. The normalized cross-correlation function is utilized to estimate the disparity based on spectral decomposition under the spatial–angular consistency of the light-field. The performance on both simulated and real captured data show that the proposed algorithm is effective at high-precision and dense disparity estimation.

Emission spectroscopy at high frame rates during ablation tests of meteorite samples in plasma wind tunnel

Optical emission spectra between 522–580 nm of ablating meteorites have been recorded at frame rates as high as 1 kHz for the first time during ground testing with simultaneous spatial and temporal resolution. A novel high frame rate emission spectroscopy arrangement has been developed and employed to diagnose the ablating meteorites in several experimental campaigns. In addition to the identification of species from emission lines detected, the resulting high-speed spectral data were used to study the temporal and spatial evolution of melting droplets and the associated spectral signatures. The time history of radiance from the atomic species emission was used to interpret the fragmentation behavior of various meteorites. Chelyabinsk meteorite exhibit almost constant radiance over time indicating steady droplet detachment whereas Ragland meteorite shows infrequent radiance peaks corresponding to random fragmentation/droplet detachment of varying sizes. A gradual rise in radiance history from iron meteorite Mount Joy shows that it takes finite time for melting and accumulation of droplets.

Flexibly Connectable Light Field System For Free View Exploration

Conventional captured-image-based virtual reality (VR) systems have three degrees of freedom (DoFs), where only rotational user motion is tracked for view rendering. This is a major cause of the reduced sense of reality. To increase user immersion levels akin to the real world, 3-DoF+ VR systems that support not only rotational but also translational view changes have been proposed. The light-field (LF) approach is suitable for this type of 3-DoF+ VR because it renders a view from a free view position by simply combining lights. Many previous systems have limited scalability because they assume a single LF for the acquisition and representation of light. One recent work connects multiple LFs at a physical intersection to increase the scalability. However, these fixed connection points limit the renderable view range and the layout of multiple LFs. Furthermore, in conventional single- or multiple-LF systems, the representable ranges of the view positions are highly dependent on the input field-of-view (FOV) of the camera used. In order to realize a wide view exploration range, the above-mentioned limitations must be overcome. This paper proposes a flexible connection scheme for multiple-LF systems taking advantage of the constant radiance of rays in LF theory. The proposed flexibly connectable LF system is able to widen the range of the renderable view position under reasonable conditions of the camera FOV. A light-field unit (LFU) which uses the proposed flexible connection is implemented. The LFU has a square-shaped structure and is thus easily stackable. This offers the advantage of dramatically expanding the scope of view explorations. The proposed LFU achieves 3-DoF+ VR with good quality as well as high scalability. Its cost-performance outcome is also better compared to those in previous works.

Effect of Radiance-Dimmer Devices Simulating Natural Sunlight Rhythm on the Plasma Melatonin Levels and Anxiety and Depression Scores of the Submarine Personnel

Objective: Not perceiving circadian shifts of sunlight due to living in enclosed environments may have deleterious effects on mental health and plasma parameters. This study aimed to determine the effect of dim regulation on the submarine personnel of Iranian Navy forces by radiating devices according to natural circadian sunlight shifts. Also, this study aimed to investigate the impact of mimicking sunlight circadian by artificial radiance luminating devices on the serological and psychological measures of submarine personnel. Method : Participants were randomly assigned to experimental and control groups in this non-randomized controlled trial. There were 26 participants in each group, and they were all male aged 21-29 years. Both groups were living in the submarine underground hall, with 120 meters 2 area with constant radiance with the same intensity. The experimental group had been given extra lighting devices with changing radiance intensity according to the natural sunlight circadian cycles. Plasma melatonin levels and depression and anxiety scores were determined before and after the experiment for both groups. Minnesota Multiphasic Personality Inventory (MMPI) and Cattell's Anxiety Scale Questionnaire (IPAT) were used to measure depression and anxiety, respectively. Results: Findings indicate that the plasma melatonin levels (-16.2±13.6 vs 8.0±9.3 mg/dL, respectively; p<0.001), depression scores (-6±6 vs 3.9±5.4, respectively; p<0.001), and anxiety scores (-1±1.2 vs 0.73±1.04, respectively; p<0.001) significantly reduced in the experimental group compared to the control group. Conclusion: Using radiance dimmers, with a radiance intensity regularity according to the sunlight, is effective in improving psychiatric and plasma parameters and can be used in closed occupational environments such as underground environments and submarine halls.

Single image dehazing for visible remote sensing based on tagged haze thickness maps

ABSTRACTHaze degrades the quality of optical remote sensing data and reduces the accuracy of interpretation and classification. In this paper, we present an empirical single image-based haze removal method. Our work relies on an additive haze model, which describes at-satellite acquired radiance as the sum of globally constant path radiance, surface reflected radiance and spatially varying haze contribution. First, we search a tagged haze thickness map in band blue, green, red and near-infrared. Then a proportional strategy is adopted to infer haze thickness map of selected starting band from tagged haze thickness map and gain haze thickness map of other spectral bands from the starting band. Finally, haze is removed through subtracting haze thickness map from each band. The method is applicable for medium- and high- resolution satellite images and can handle scenes affected by different haze depths. Dehazed results are in good quality with improved visibility, enhanced image information and highly spectral consistency. The algorithm is simple enough and requires no human intervention, making it possible for use of non-experts and implementation of automatic batch processing.

Diffuse back-illumination setup for high temporally resolved extinction imaging.

This work presents the development of an optical setup for quantitative, high-temporal resolution line-of-sight extinction imaging in harsh optical environments. The application specifically targets measurements of automotive fuel sprays at high ambient temperature and pressure conditions where time scales are short and perceived attenuation by refractive index gradients along the optical path (i.e., beam steering) can be significant. The illumination and collection optics are optimized to abate beam steering, and the design criteria are supported by well-established theoretical relationships. The general effects of refractive steering are explained conceptually using simple ray tracing. Three isolated scenarios are analyzed to establish the lighting characteristics required to render the observed radiant flux unaffected by the steering effect. These criteria are used to optimize light throughput in the optical system, enabling minimal exposure times and high-temporal resolution capabilities. The setup uses a customized engineered diffuser to transmit a constant radiance within a limited angular range such that radiant intensity is maximized while fulfilling the lighting criteria for optimal beam-steering suppression. Methods for complete characterization of the optical system are detailed. Measurements of the liquid-vapor boundary and the soot volume fraction in an automotive spray are presented to demonstrate the resulting improved contrast and reduced uncertainty. The current optical setup reduces attenuation caused by refractive index gradients by an order of magnitude compared to previous high-temporal resolution setups.

REVIEW OF THRESHOLDS AND RECOMMENDATIONS FOR REVISED EXPOSURE LIMITS FOR LASER AND OPTICAL RADIATION FOR THERMALLY INDUCED RETINAL INJURY

Exposure limits (ELs) for laser and optical broadband radiation that are derived to protect the retina from adverse thermally-induced effects vary as a function of wavelength, exposure duration, and retinal irradiance diameter (spot size) expressed as the angular subtense α. A review of ex vivo injury threshold data shows that, in the ns regime, the microcavitation-induced damage mechanism results in retinal injury thresholds below thermal denaturation-induced thresholds. This appears to be the reason that the injury thresholds for retinal spot sizes of about 80 μm (α = 6 mrad) and pulse durations of about 5 ns in the green wavelength range are very close to current ELs, calling for a reduction of the EL in the ns regime. The ELs, expressed in terms of retinal radiant exposure or radiance dose, currently exhibit a 1/α dependence up to a retinal spot size of 100 mrad, referred to as αmax. For α ≥ αmax, the EL is a constant retinal radiant exposure (no α dependence) for any given exposure duration. Recent ex vivo, computer model, and non-human primate in vivo threshold data provide a more complete assessment of the retinal irradiance diameter dependence for a wide range of exposure durations. The transition of the 1/α dependence to a constant retinal radiant exposure (or constant radiance dose) is not a constant αmax but varies as a function of the exposure duration. The value of αmax of 100 mrad reflects the spot size dependence of the injury thresholds only for longer duration exposures. The injury threshold data suggest that αmax could increase as a function of the exposure duration, starting in the range of 5 mrad in the μs regime, which would increase the EL for pulsed exposure and extended sources by up to a factor of 20, while still assuring an appropriate reduction factor between the injury threshold and the exposure limit.

Atmospheric correction and surface spectral unit mapping using Thermal Emission Imaging System data

Three techniques are described for the extraction of surface emissivity and compositional information from Thermal Emission Imaging System (THEMIS) images. Synthetic images were constructed with different atmospheric properties and random and systematic errors to estimate the uncertainty in the retrieved surface emissivity. The three techniques are as follows: (1) A constant radiance removal algorithm determines and removes a constant radiance from atmospheric emission as well as systematic calibration radiance offsets. (2) Surface emissivity retrieval uses low‐resolution Thermal Emission Spectrometer surface emissivity data to determine atmospheric properties for an image that can be then applied to individual THEMIS pixels. (3) A spectral unit mapping algorithm determines spectral end‐member concentrations using a deconvolution routine similar to several previous applications. The initial application of these techniques to three images covering the same region of the Martian surface but at different surface temperature and atmospheric conditions yields consistent surface spectral shapes as well as end‐member concentrations. Retrieved aerosol opacity information is consistent with an independently developed opacity retrieval method. The application of the techniques described here is done in a stepwise fashion and may be applied to the desired level of analysis necessary for interpretation of surface properties.

Multiplex sensors and the constant radiance theorem

Coherent mode representation of the cross-spectral density is used to derive a modal analog of the constant radiance theorem with general applicability to linear optical systems. The theorem is used to consider the relationship between spatial detector geometry and multiplexing capacity.

Analysis of underwater radiance observations: Apparent optical properties and analytic functions describing the angular radiance distribution

A primary data set consisting of 70 series of angular radiance distributions observed in clear blue western Mediterranean water and a secondary set of 12 series from the more green and turbid Lake Pend Oreille, Idaho, have been analyzed. The results demonstrate that the main variation of the shape of the downward radiance distribution occurs within the Snell cone. Outside the cone the variation of the shape decreases with increasing zenith angle. The most important shape changes of the upward radiance appear within the zenith angle range 90°–130°. The variation in shape reaches its minimum around nadir, where an almost constant upward radiance distribution implies that a flat sea surface acts like a Lambert emitter within ±8% in the zenith angle interval 140°–180° in air. The ratio Q of upward irradiance and nadir radiance, as well as the average cosines μd and μu for downward and upward radiance, respectively, have rather small standard deviations, ≤10%, within the local water type. In contrast, the irradiance reflectance R has been observed to change up to 400% with depth in the western Mediterranean, while the maximum observed change of Q with depth is only 40%. The dependence of Q on the solar elevation for blue light at 5 m depth in the Mediterranean coincides with observations from the central Atlantic as well as with model computations. The corresponding dependence of μd shows that diffuse light may have a significant influence on its value. Two simple functions describing the observed angular radiance distributions are proposed, and both functions can be determined by two field observations as input parameters. The ε function approximates the azimuthal means of downward radiance with an average error ≤7% and of upward radiance with an error of ∼1%. The α function describes the zenith angle dependence of the azimuthal means of upward radiance with an average error ≤7% in clear ocean water, increasing to ≤20% in turbid lake water. The a function suggests that the range of variation for μu falls between 0 and 1/2, and for Q it is between π and 2π. The limits of both ranges are confirmed by observations. By combining the ε and α functions, a complete angular description of the upward radiance field is achieved.

RX: a nonimaging concentrator

A detailed description of the design procedure for a new concentrator, RX, and some examples of it's use are given. The method of design is basically the same as that used in the design of two other concentrators: the RR and the XR [Appl. Opt. 31, 3051 (1992)]. The RX is ideal in two-dimensional geometry. The performance of the rotational RX is good when the average angular spread of the input bundle is small: up to 95% of the power of the input bundle can be transferred to the output bundle (with the assumption of a constant radiance for the rays of the input bundle).

Equilibrium radiance model applications and comparisons to atmospheric measurements and Rayleigh models

The equilibrium radiance model, which assumes constancy in the source function along the path of sight, predicts equilibrium radiance as a function of the angle from the sun. Measurements made at all altitudes in the troposphere of cloudless sky radiance in the visible spectrum consistently support this prediction. As a result, sky radiances can be used to obtain (1) vertical space-to-sensor scattering transmittance, (2) equilibrium radiance, and (3) path radiance through the atmosphere and in the troposphere. Comparisons are made of the constant equilibrium radiance model predictions of sky radiance, irradiance, and equilibrium radiance to (1) other models for the Rayleigh atmosphere, and (2) to measurements in the troposphere for thin atmospheres. The agreements are good.

Scene-Based Nonuniformity Compensation for Imaging Sensors

Multidetector imaging focal plane arrays like CCD TV cameras and infrared arrays possess large detector-to-detector dark current (offset) and responsivity (gain) variations which can completely mask the useful information. Conventional compensation techniques require temperature references of constant radiance over the entire field of view and a mechanical/electrooptical shutter to calibrate the focal plane. This detracts from the mechanical simplicity of multi-detector staring (nonscanning) focal planes. This correspondence describes a real-time offset and responsivity (gain) compensation technique which dispenses with temperature references and shutters in staring focal planes. It makes use of the scene statistics for calibration and continuously updates the compensation coefficients without interrupting the field of view. The results of real-time simulations of this technique with a number of sensors are presented. Real-time LSI/VLSI hardware architectures are addressed. The technique can be implemented with the addition of very little hardware to a conventional compensation technique requiring temperature references. The technique is also suitable for multi-detector scanning focal planes and for the removal of shading in TV sensors as well.

Theory of the sunpillar

AbstractThe radiance distribution predicted by classical physical models of the sunpillar is calculated. It is shown that the loci of constant tip angle calculated by Stuchtey differ from loci of constant radiance only as a result of variation of the reflection coefficient with arc distance from the source. At least at low source elevations, crystals oscillating about a horizontal equilibrium position are more efficient as pillar producers than those rotating about horizontal C6 axes, but there is no conclusive evidence that the latter never produce pillars. It is shown that the variation of the cross‐sectional area of the incident beam has important effects, such as production of a minimum radiance at the source. Neglect of this factor by recent workers casts doubt on some of their conclusions.No evidence is found to suggest that the classical models need replacement: tentative suggestions are made that diffraction, transfer effects, and modes other than external reflection in crystals rotating about horizontal C6 axes may enhance the formation of pillars at higher source elevations.

Apparent saturation of blue-sensitive cones occurs at a color-opponent stage.

Response saturation of blue-sensitive cone pathways was studied by measuring increment thresholds for violet test flashes on flashed violet fields in the presence of a steady yellow "auxiliary" field of constant radiance. Adding intense yellow field flashes to the violet field flash could eliminate or reduce response saturation (greatly reduce threshold), whereas "negative" yellow field flashes drove the mechanism to further saturation. The response saturation is thus not, in general, controlled exclusively by independent blue-sensitive cones but by spectrally opponent mechanisms that receive opposite-signed signals from blue-sensitive cones and from green-or red-sensitive cones. These results add to a growing number of studies that demonstrate that detection of signals from blue-sensitive cones is largely through a color-opponent pathway.

Reconstruction of Objects from their Diffraction Images

A relatively simple theory for reconstructing self-luminous objects from their diffraction images is presented for thin object disks that have axial symmetry and for thin, long objects whose radiance varies only along the width. Availability of a priori information about the variation of the radiance is assumed. With these simple objects the complete spatial-frequency spectrum of the object function can be deduced in a direct manner from considerations of the Fourier transform of the measured irradiance in an image having negligible noise. Objects having constant radiance are treated in detail and are found to be advantageous for use in experiments designed to test the validity and practicality of the principles of object reconstruction.